Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2019
Allowance for Credit Losses
Beginning balance	$ 1,179,021	$ 1,361,723	$ 1,673,324	$ 1,361,723	$ 441,912
Transfer from unfunded commitment liability			37,010	151,436	179,085
Provision for current period	1,194		258,626	(253,805)	428,134
Loan sale reduction to provision	(1,477)		0	(10,335)	(161,793)
Loan transfer from held-for-sale				1,887
Total provision	283		(258,626)	262,253	(266,341)
Net charge-offs:
Charge-offs	(50,028)		(46,505)	(105,344)	(117,454)
Recoveries	6,978		6,868	14,682	16,386
Net charge-offs	(43,050)		(39,637)	(90,662)	(101,068)
Ending Balance	1,160,244	1,179,021	1,929,323	1,160,244	1,929,323
Allowance
Ending balance: individually evaluated for impairment	80,495		160,234	80,495	160,234
Ending balance: collectively evaluated for impairment	1,079,749		1,769,089	1,079,749	1,769,089
Loans:
Ending balance: individually evaluated for impairment	1,192,743		1,520,240	1,192,743	1,520,240
Ending balance: collectively evaluated for impairment	20,014,756		21,502,357	20,014,756	21,502,357
Provisions for loan losses	(463)		234,015	(262,376)	216,084
Provisions for unfunded loan commitments	69,960		93,261	106,163	146,804
Total Private Education Loan provisions for credit losses	69,497		327,276	(156,213)	362,888
Private Education Loan provision for credit losses, other impacts	180		24,611	123	50,257
Less: provisions for credit losses	69,677		351,887	(156,090)	413,145
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance					1,584,965
Day 1 adjustment for the adoption of CECL						$ 1,143,053
Transfer from unfunded commitment liability	24,556
FFELP Loans
Allowance for Credit Losses
Beginning balance	4,318	4,378	4,296	4,378	1,633
Transfer from unfunded commitment liability			0	0	0
Provision for current period	(2)		173	27	210
Loan sale reduction to provision	0		0	0	0
Loan transfer from held-for-sale				0
Total provision	2		(173)	(27)	(210)
Net charge-offs:
Charge-offs	(54)		(84)	(143)	(310)
Recoveries	0		0	0	0
Net charge-offs	(54)		(84)	(143)	(310)
Ending Balance	4,262	4,318	4,385	4,262	4,385
Allowance
Ending balance: individually evaluated for impairment	0		0	0	0
Ending balance: collectively evaluated for impairment	4,262		4,385	4,262	4,385
Loans:
Ending balance: individually evaluated for impairment	0		0	0	0
Ending balance: collectively evaluated for impairment	$ 716,958		$ 754,340	$ 716,958	$ 754,340
Net charge-offs as a percentage of average loans in repayment (annualized)	0.04%		0.07%	0.05%	0.11%
Allowance as a percentage of the ending total loan balance	0.59%		0.58%	0.59%	0.58%
Allowance as a percentage of the ending loans in repayment	0.78%		0.97%	0.78%	0.97%
Allowance coverage of net charge-offs (annualized)	19.73		13.05	14.90	7.07
Ending total loans, gross	$ 716,958		$ 754,340	$ 716,958	$ 754,340
Average loans in repayment	546,306		513,418	551,765	553,402
Ending loans in repayment	548,488		452,617	548,488	452,617
Private Education Loan provision for credit losses, other impacts	(2)		173	27	210
FFELP Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance					4,485
Day 1 adjustment for the adoption of CECL						2,852
Transfer from unfunded commitment liability	0
Private Education Loans
Allowance for Credit Losses
Beginning balance	1,173,375	1,355,844	1,515,781	1,355,844	374,300
Transfer from unfunded commitment liability			37,010	151,436	179,085
Provision for current period	1,014		234,015	(253,928)	377,877
Loan sale reduction to provision	(1,477)		0	(10,335)	(161,793)
Loan transfer from held-for-sale				1,887
Total provision	463		(234,015)	262,376	(216,084)
Net charge-offs:
Charge-offs	(49,900)		(31,779)	(105,039)	(83,248)
Recoveries	6,972		5,532	14,675	13,508
Net charge-offs	(42,928)		(26,247)	(90,364)	(69,740)
Ending Balance	1,154,540	1,173,375	1,760,559	1,154,540	1,760,559
Allowance
Ending balance: individually evaluated for impairment	80,495		160,234	80,495	160,234
Ending balance: collectively evaluated for impairment	1,074,045		1,600,325	1,074,045	1,600,325
Loans:
Ending balance: individually evaluated for impairment	1,192,743		1,520,240	1,192,743	1,520,240
Ending balance: collectively evaluated for impairment	$ 19,285,014		$ 19,965,225	$ 19,285,014	$ 19,965,225
Net charge-offs as a percentage of average loans in repayment (annualized)	1.16%		0.75%	1.23%	0.91%
Allowance as a percentage of the ending total loan balance	5.64%		8.19%	5.64%	8.19%
Allowance as a percentage of the ending loans in repayment	7.79%		12.13%	7.79%	12.13%
Allowance coverage of net charge-offs (annualized)	6.72		16.77	6.39	12.62
Ending total loans, gross	$ 20,477,757		$ 21,485,465	$ 20,477,757	$ 21,485,465
Average loans in repayment	14,743,360		14,011,841	14,738,505	15,306,349
Ending loans in repayment	14,825,375		14,512,723	14,825,375	14,512,723
Private Education Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance					1,435,130
Day 1 adjustment for the adoption of CECL						1,060,830
Transfer from unfunded commitment liability	24,556
Personal Loans
Allowance for Credit Losses
Beginning balance			152,673		65,877
Transfer from unfunded commitment liability			0		0
Provision for current period			23,929		49,247
Loan sale reduction to provision			0		0
Total provision			(23,929)		(49,247)
Net charge-offs:
Charge-offs			(14,601)		(33,848)
Recoveries			1,336		2,878
Net charge-offs			(13,265)		(30,970)
Ending Balance			163,337		163,337
Allowance
Ending balance: individually evaluated for impairment			0		0
Ending balance: collectively evaluated for impairment			163,337		163,337
Loans:
Ending balance: individually evaluated for impairment			0		0
Ending balance: collectively evaluated for impairment			$ 772,086		$ 772,086
Net charge-offs as a percentage of average loans in repayment (annualized)			6.87%		7.13%
Allowance as a percentage of the ending total loan balance			21.16%		21.16%
Allowance as a percentage of the ending loans in repayment			22.46%		22.46%
Allowance coverage of net charge-offs (annualized)			3.08		2.64
Ending total loans, gross			$ 772,086		$ 772,086
Average loans in repayment			772,137		869,133
Ending loans in repayment			727,214		727,214
Private Education Loan provision for credit losses, other impacts			23,929		49,247
Personal Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance					145,060
Day 1 adjustment for the adoption of CECL						79,183
Credit Cards
Allowance for Credit Losses
Beginning balance	1,328	1,501	574	1,501	102
Transfer from unfunded commitment liability			0	0	0
Provision for current period	182		509	96	800
Loan sale reduction to provision	0		0	0	0
Loan transfer from held-for-sale				0
Total provision	(182)		(509)	(96)	(800)
Net charge-offs:
Charge-offs	(74)		(41)	(162)	(48)
Recoveries	6		0	7	0
Net charge-offs	(68)		(41)	(155)	(48)
Ending Balance	1,442	$ 1,328	1,042	1,442	1,042
Allowance
Ending balance: individually evaluated for impairment	0		0	0	0
Ending balance: collectively evaluated for impairment	1,442		1,042	1,442	1,042
Loans:
Ending balance: individually evaluated for impairment	0		0	0	0
Ending balance: collectively evaluated for impairment	$ 12,784		$ 10,706	$ 12,784	$ 10,706
Net charge-offs as a percentage of average loans in repayment (annualized)	2.29%		1.77%	2.59%	1.31%
Allowance as a percentage of the ending total loan balance	11.28%		9.73%	11.28%	9.73%
Allowance as a percentage of the ending loans in repayment	11.28%		9.73%	11.28%	9.73%
Allowance coverage of net charge-offs (annualized)	5.30		6.35	4.65	10.85
Ending total loans, gross	$ 12,784		$ 10,706	$ 12,784	$ 10,706
Average loans in repayment	11,855		9,265	11,964	7,326
Ending loans in repayment	12,784		10,706	12,784	10,706
Private Education Loan provision for credit losses, other impacts	182		$ 509	$ 96	800
Credit Cards | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance					$ 290
Day 1 adjustment for the adoption of CECL						$ 188
Transfer from unfunded commitment liability	$ 0